The present invention relates generally to comparators, and more particularly to differential comparators having controllable threshold voltages and/or controllable hysteresis.
FIG. 1 shows a conventional comparator 1 that includes an input stage 10 with N-channel input transistors MN2 and MN3 having their gates coupled to receive input voltages Vin− and Vin+, respectively, and having their sources connected to one terminal of a bias current source 16, the other terminal of which is connected to VSS. The drain of transistor MN2 is connected by conductor 2 to the gate and drain of P-channel current mirror input transistor MP10 and to the gate of P-channel current mirror output transistor MP9, the sources of which are connected to VDD. The drain of current mirror output transistor MP9 is connected by conductor 3 to the drain of input transistor MN3. Comparator 1 also includes an output stage 6 which includes P-channel input transistors MP13 and MP14 having their sources connected to VDD and having their gates connected to conductors 2 and 3, respectively. The drain of transistor MP13 is connected by conductor 4 to the drain and gate of N-channel current mirror input transistor MN0 and the gate of N-channel current mirror output transistor MN1, the sources of which are connected to VSS. The drain of current mirror output transistor MN1 is connected by conductor 5 to the drain of transistor MP14 and to the input of an inverter 7. The output of inverter 7 is connected by conductor 9 to the input of an inverter 8 and also to the gate of a N-channel hysteresis feedback transistor MN27, the drain of which is connected to conductor 2 and the source of which is connected to one terminal of a hysteresis current source 18 having its other terminal connected to VSS.
Transistor MN27 performs the function of providing positive feedback to introduce hysteresis into the comparator. The hysteresis current I1 is much lower than the bias current I0. The ratio of the two currents I1 and I0 determines the amount of hysteresis. Inverters 7 and 8 provide additional gain to sharpen the edges of the comparator output signal.
The switching threshold voltage, or simply the “threshold”, of a comparator is the value of the input voltage or differential input voltage at which the comparator changes state. The hysteresis of a comparator is the difference between the input voltage at which a comparator switches in one direction and the input voltage at which the comparator switches in the opposite direction.
The threshold voltage of conventional differential comparator 1 of FIG. 1 is equal or very nearly equal to zero. The circuit shown in FIG. 1 changes state when the differential input voltage is very close to or exactly equal to zero. It has a non-symmetrical hysteresis created by injecting the typically relatively small current I1 into the input of a current mirror load circuit MP9,MP10.
By way of background, it should be noted that the technique of using two or more pairs of differentially coupled input transistors and their associated tail current sources has been commonly used in design of various amplifiers, for example to provide various ways of auto-zeroing, but it is believed that this technique has not been used to establish an adjustable switching threshold voltage and/or an adjustable amount of hysteresis in a comparator.
If a conventional comparator, for example comparator 1 of FIG. 1, is used as a component in a system, there frequently is a need for it to have a well controlled threshold that is different from zero. For example, a window comparator in an ADC (analog to digital converter) typically is formed from two comparators, one with a positive threshold and the other with a negative threshold. It would be desirable to be able to precisely control the threshold and/or hysteresis of a comparator, because that could significantly simplify the electronic circuitry and provide functions that otherwise would require additional sub-modules. For example, it may be desirable to have a comparator that changes state at a threshold of approximately 100 millivolts. Or it may be desirable to have a window comparator that, in effect, has 3 states, namely +1, 0, and −1, such that the comparator switches to the “+1” state when the differential input voltage is above +100 millivolts, and can be considered to be in the “0” condition when the differential input voltage is between +100 millivolts and −100 millivolts, and can be considered to be in the “−1” state when the differential input voltage is below −100 millivolts.
Since conventional comparators only have a threshold that is very close to or exactly equal to 0 volts, the only known practical way to provide a window comparator has been to modify the thresholds of its two conventional comparators by scaling the geometries of various transistors therein such that one of the comparators changes state when the differential input voltage is at, for example, +100 millivolts and the other comparator changes state when the differential input voltage is at, for example, −100 millivolts. This approach has the shortcoming that it results in wide threshold and/or hysteresis variations with respect to temperature changes and/or manufacturing process variations. And of course, once a particular threshold is designed into a conventional comparator by scaling various transistor geometries, it cannot be easily changed.
Most known hysteresis circuits in comparators use a signal from the comparator output to control a voltage or current inside the comparator in one direction or the other so as to imbalance the circuit to provide a desired fixed amount of hysteresis.
There is an unmet need for a comparator in which a non-zero threshold can be achieved without mismatching various transistor geometries therein.
There also is an unmet need for a comparator in which the threshold can be adjusted.
There also is an unmet need for a comparator having adjustable hysteresis.
There also is an unmet need for a comparator having adjustable threshold and adjustable hysteresis.
There also is an unmet need for a comparator and method wherein the threshold can be varied during comparator operation.
There also is an unmet need for a comparator and method wherein both the threshold and hysteresis of the comparator can be adjusted during comparator operation.